Many thanks to commenter Craig for helping me place my finger on this exact quote, which is worthy of repeating both for its keen insight and its classic Lewis tone.
Every age has its own outlook. It is specially good at seeing certain truths and specially liable to make certain mistakes. We all, therefore, need the books that will correct the characteristic mistakes of our own period. And that means the old books. All contemporary writers share to some extent the contemporary outlook – even those, like myself, who seem most opposed to it. Nothing strikes me more when I read the controversies of past ages than the fact that both sides were usually assuming without question a good deal which we should now absolutely deny. They thought that they were as completely opposed as two sides could be, but in fact they were all the time secretly united – united with each other and against earlier and later ages – by a great mass of common assumptions. We may be sure that the characteristic blindness of the twentieth [or twenty-first] century – the blindness about which posterity will ask, ‘But how could they have thought that?’ – lies where we have never suspected it… None of us shall fully escape this blindness, but we shall certainly increase it, and weaken our guard against it, if we read only modern books… The only pallative is to keep the clean sea breeze of the centuries blowing through our minds, and this can be done only by reading old books. Not, of course, that there is any magic about the past. People were no cleverer then than they are now; they made as many mistakes as we. But not the same mistakes.
– C.S. Lewis, Introduction to St. Athanasius’ On The Incarnation (Crestwood, NY: St. Vladimir’s Press, 2002; orig. ed. Centenary Press, 1944).